In pressurized water nuclear reactors for the production of power, a pressurized fluid, such as water, is circulated as a primary coolant through the reactor core and heat transferred from the primary coolants, through heat exchange tubes in the secondary side of a steam generator, to a secondary coolant flowing through the secondary side. In the secondary side of the steam generator, the heat so transferred produces steam that is then used to drive a turbine for the production of electrical power.
The steam generator has a main feedwater line from which secondary coolants, such as water, is fed to the secondary side of the steam generator for conversion to steam, and a steam dicharge line through which steam produced is directed to the turbine. An emergency feedwater line is also provided so as to transmit emergency secondary coolant to the steam generator following an accident or during a transient condition when the main feedwater system is not available, so as to provide a supplemental source of secondary coolant to the steam generator.
The steam generators are also provided with a blowdown system. This system is used to eliminate the concentration of impurities in the secondary side of the steam generator and normally comprises a blowdown discharge line that will discharge about one percent of the secondary coolant to a heat exchanger and then to a condensate demineralizer to remove impurities and then to a condensate tank for subsequent return to the main feedwater line to the steam generator.
In the event of a need to provide a cold shutdown of the reactor, residual heat and sensible heat contained in the core must be removed. Conventionally, a reactor coolant system is used, in conjunction with the steam generators so as to bring the plant temperature from a no load temperature to cold shutdown condition. Such a cold shutdown was normally to be accomplished in two phases; first, a transfer of reactor coolant system heat to the steam system, such as by the release of steam to the atmosphere or to a condenser in the steam system, in order to reduce the temperature of the primary coolant from about 285.degree. C. to 175.degree. C. within four hours after reactor shutdown; and second, initiation of a residual heat removal system that is conventionally provided so as to reduce the temperature to about 60.degree. C. within twenty hours following the reactor shutdown. Under new regulations, the shutdown system must be capable of meeting cold shutdown as a safe plant condition after accident mitigation, and the use of safety grade equipment is required.
An object of the present invention is to provide a steam generator recirculating system that is usable to bring the reactor from a no load condition to cold shutdown condition with safety grade equipment.
Another object of the present invention is to provide a steam generator recirculating system that will recirculate the secondary coolant from the steam generator of a pressurized water reactor system to achieve uniform water chemistry.
A further object of the present invention is to provide a steam generator recirculating system that can also be used as a steam generator blowdown system or a water purification means to remove impurities from the secondary coolant water.